Physically sound model of a non-Foster negative capacitor

In both engineering and physics communities, it is believed that approximation of a realistic non-Foster negative capacitor (within its operating bandwidth) with an ideal dispersionless negative capacitor is acceptable for practical purposes. However, the ideal negative capacitor is not causal and, therefore, not compatible with basic physics. Its use in the design process is misguiding since it often predicts entirely non-physical behaviour. Here, we show that a realistic negative capacitor can always be modelled as a dispersive voltage-controlled source, the internal impedance of which is an ordinary positive capacitor. This equivalent circuit clearly explains the origin of negative conductance that always accompanies negative capacitance, as well as the background physics of previously reported counter-intuitive phenomena in nonFoster metamaterials. Theoretical analysis was verified by simulations and measurements on an experimental low-frequency (100 Hz–25 kHz) negative capacitor demonstrator. Measurement results agreed well with theoretical predictions, showing that the proposed model is indeed physically sound